1. Field of the Invention
This invention relates to a process for the preparation of beta-diketones and, more particularly, to a process for the production of beta-diketones by means of a homogeneous or nearly homogeneous liquid reaction medium wherein a carboxylic acid ester is reacted with a ketone in the presence of a condensing agent that is a sterically hindered alkali metal alkoxide dissolved in an aromatic hydrocarbon.
2. Description of Related Art
Beta-diketones are highly valuable compounds having advantageous utility in a wide variety of applications. For example, U.S. Pat. No. 3,001,970 discloses the use of dibenzoylmethane to prevent the discoloration of vinylidene chloride. U.S. Pat. No. 3,493,536 discloses that diaroylmethane compounds provide stabilizing action against the sensitizing effect of bismuth or antimony compounds on chlorine containing materials. Aryl substituted beta-diketones are shown by U.S. Pat. No. 3,994,869 to be useful as accelerators for the photodegradation of polyolefins. U.S. Pat. No. 4,427,816 discloses beta-diketones in combination with hydrotalcites as stabilizer compositions for halogen containing polymers.
The preparation of beta-ketones is reported in Organic Reactions (Vol. 8, 1959), Chapter 3, (pages 59-195) entitled xe2x80x9cThe Acylation of Ketones to Form xcex2-Diketones or xcex2-Keto Aldehydes.xe2x80x9d The article states in its introduction at page 61, xe2x80x9cUnder certain conditions, a ketone having an xcex1-hydrogen atom may be acylated with an ester, an acid anhydride, or an acid chloride to form a xcex2-diketone or, when the acylating agent is a formic ester, xcex2-keto aldehyde. The process consists in the replacement of an xcex1-hydrogen atom of the ketone by an acyl group; . . .xe2x80x9d
Unfortunately, the acylation of ketones, as achieved by using previously known procedures, is a reaction that does not readily proceed in an economical manner. On page 66 of this same text, for example, it is pointed out that the acylation of ketones with esters in the presence of a basic reagent may be accompanied by certain side reactions. Among the side reactions that may occur are self-condensation of the ketone, self-condensation of the ester, aldol reaction of the ester with the carbonyl group of the ketone, or a Michael condensation of the ketone. Also, the basic condensing agent may react with the carbonyl group of the ester.
The preparation on a laboratory scale of dibenzoyl methane by the reaction of acetophenone and ethyl benzoate in the presence of sodium ethoxide and the absence of solvent is reported by Magnani and McElvain in Organic Synthesis, Collective Volume 3, pp. 251-253. This reaction used 4 moles of ethyl benzoate and 0.5 mole of acetophenone. The reaction mixture was gelatinous after all of the ethoxide had been added and was too viscous to be stirred with a Hershberg stirrer. The yield of dibenzoyl methane recovered from the reaction mixture, as reported, was 62-71% based on the acetophenone.
In general, the reaction of the ester, the ketone, and the basic condensing agent in the presence of an inert solvent is known, as in the aforementioned Organic Reactions article at page 112. This article further states that the beta-diketone may be isolated by the usual technique of distillation or filtration, but often it is isolated as its copper derivative from which the beta-diketones need to be regenerated by further chemical reactions with concomitant yield losses, generation of waste products, and laborious recovery of copper.
The use of copper derivatives is an expensive and environmentally undesirable procedure. Furthermore, the occurrence of side reactions would prevent the commercial success of the process. Then, too, loss of solvent and the need to use fresh solvent for each reaction is commercially unattractive.
U.S. Pat. No. 3,994,869, mentioned above, discloses the preparation of aryl substituted beta-diketones by the reaction of acetophenone or a substituted acetophenone with an ester in the presence of a base, such as sodium methoxide, sodium ethoxide, and sodium hydride. The acetophenone may be represented by the structural formula: 
wherein R is selected from the group consisting of hydrogen, halogen, C1 to C9 alkyl and C1 to C9 alkoxy. Representative esters identified as useful in this reaction are methyl stearate, ethyl benzoate, ethyl acetate, and ethyl laurate. This reaction, according to the patent, can be carried out in a suitable aprotic solvent, such as toluene or tetrahydrofuran. Recovery of the desired product is stated to be by methods that are now known in the art.
While it has been known that beta-diketones can be made by the reaction of acetophenone or a substituted acetophenone with an ester in the presence of base, this procedure has drawbacks which have limited its commercial acceptability. U.S. Pat. No. 4,482,745 discloses that handling large quantities of strong bases, such as sodium ethoxide makes their use undesirable and costly for large scale production. Yet, sodium alkoxides are preferred bases since only one mole is consumed, whereas two moles of metallic sodium, sodium amide, or sodium hydride would normally be required. Then, too, metallic sodium or sodium hydride are more hazardous than the alkoxides. Aromatic beta-diketones in high yields and purity can be readily made by this process. The method taught by this patent comprises reacting acetophenone with from 5 to 10 molar equivalents of methyl benzoate in the presence of from 1 to 2 molar equivalents of calcium oxide, in a temperature range of from 150xc2x0 to 200xc2x0 C. for from three to six hours under an inert nitrogen atmosphere while continuously removing the methyl alcohol which is formed during the reaction.
U.S. Pat. No. 5,015,777 and European Patent No. 0 507 013 B1 disclose a process for the preparation of aromatic beta-diketones by the reaction of an acetophenone and a molar excess of an alphatic ester or an ester of benzoic acid in the presence of sodium alkoxide condensation agent in an aromatic hydrocarbon solvent. Also disclosed is a method of recycling the solvent and excess ester reactant after separation of the aromatic beta-diketone product.
U.S. Pat. No. 5,344,992 and European Patent No. 0 454 624 B1 disclose a process for the preparation of 1,3-diketones of formula I 
wherein R1 and R2 are each independently of the other C1-C20 alkyl, phenyl or phenyl which is substituted by halogen, hydroxy, NO2, C1-C4 alkyl and/or C1-C4 alkoxy, C7-C9 phenylalkyl or a radical of formula II
xe2x80x94Axe2x80x94Xxe2x80x94R4xe2x80x83xe2x80x83(II)
wherein A is C1-C12 alkylene, phenylene or phenylene which is substituted by halogen, hydroxy, NO2, C1-C4 alkyl and/or C1-C4 alkoxy, or is C1-C12 alkylene which is substituted by hydroxy, halogen and/or alkoxy, X is oxygen or sulfur, and R4 is hydrogen, C1-C18 alkyl, phenyl or phenyl which is substituted by halogen, hydroxy, C1-C4 alkyl, NO2 and/or C1-C4 alkoxy, or is C7-C9 phenylalkyl, and R3 is hydrogen, C1-C20 alkyl, phenyl or phenyl which is substituted by halogen, hydroxy, C1-C4 alkyl, NO2 and/or C1-C4 alkoxy, or is C7-C9 phenylalkyl. The process comprises carrying out a Claisen condensation of a ketone of fornula III 
and an ester of formula IV 
or 
wherein m is 2 to 10 and R5 is C1-C5 alkyl, phenyl or phenyl which is substituted by halogen, C1-C4 alkyl or hydroxy, the reaction being carried out with the base used as catalyst, a hydride of an alkali metal or alkaline earth metal or an alcoholate of C1-C5 alkali metal or C1-C5 alkaline earth metal, in a mixture of dimethyl sulfoxide and at least one organic solvent which is inert under the reaction conditions. The use of dimethyl sulfoxide is a disadvantage owing to the difficulty of completely removing it from the product and of recovering it for reuse without significant losses.
U.S. Pat. No. 5,672,646 discloses a stabilizing composition for a chlorine-containing polymer (PVC), characterized in that it comprises the unpurified crude product resulting from the reaction of an ester with a ketone in the presence of an alkaline agent, this crude product comprising at least 10% by weight of xcex2-diketone and being in the form of a powder.
U.S. Pat. No. 5,808,165 discloses compositions containing beta-diketones of formula (I) and formula (II),
R1COCH2COR2xe2x80x83xe2x80x83(I)
R2COCH2COR2xe2x80x83xe2x80x83(II)
which may be used to stabilize various polymers, such as polyvinyl chlorides (PVCs), in which R1 is represented by the formula
(Y)nxe2x80x94"PHgr"xe2x80x94,
wherein "PHgr" is phenyl and each Y, which may be the same or different, is a hydrogen atom or a group selected from hydrocarbon chains having 1 to 12 carbon atoms, alkoxys, silyls and nonreactive halogen atoms; each R2, which may be the same or different, represents a hydrogen atom or a group selected from hydrocarbon chains having 1 or 5 to 12 carbon atoms, which may be interrupted by one or more oxygen atoms, aralkyls, alkoxys and silyls; and n represents an integer from 0 to 3; with the proviso that if the number of carbon atoms in R2 in formula (1) is less than 5, the sum of the carbons contained in Y is at least 3 and at most 12, and that in formula (II) the total number of carbon atoms in the two R2""s is at least 10.
Dibenzoylmethane (DBM) is currently produced commercially by means of a Claisen condensation of acetophenone and methyl benzoate in the solvent cumene with sodium methoxide being used as the base. This process, however, presents several problems:
1) the sodium methoxide is a very fine powder that is very reactive toward water and carbon dioxide from air and is extremely hard to handle;
2) sodium methoxide is insoluble in cumene, which complicates the production process; and
3) sodium methoxide is, after acetophenone, the second most expensive raw material for the manufacture of DBM.
Thus, an improved process for making 1,3-diketones is still needed by industry.
The disclosures of the foregoing are incorporated herein by reference in their entirety.
In accordance with this invention, there is provided a process of preparing a 1,3-diketone from a carboxylic acid ester and a ketone by a condensation reaction promoted by a basic condensing agent that overcomes the problems outlined above by using a novel basic condensing agent that is soluble in an aromatic hydrocarbon and is an alkali metal alkoxide of an alcohol having a boiling point of at least 100xc2x0 C. at atmospheric pressure. By the use of this condensing agent according to the invention, the handling of reagents has been simplified, side reactions during condensation are minimized, and the yields of 1,3-diketone are increased significantly.
There is also provided, in accordance with this invention, a process for preparing the novel basic condensing agent by heating a mixture consisting essentially of an alcohol having a boiling point of at least 100xc2x0 C. at atmospheric pressure, an alkali metal hydroxide, and an aromatic hydrocarbon, and removing water formed in the reaction of the alcohol with the alkali metal hydroxide.
There is, moreover, provided in accordance with this invention, a basic condensing agent consisting essentially of a solution of alkali metal alkoxide of an alcohol having a boiling point of at least 100xc2x0 C. at atmospheric pressure in an aromatic hydrocarbon. The expression xe2x80x9cconsisting essentially ofxe2x80x9d is used to indicate that polar aprotic solvents such as dimethyl sulfoxide are unnecessary and objectionable according to this invention.
When the two processes according to the invention are combined, the alcohol used in preparing the basic condensing agent is regenerated in the preparation of the 1,3-diketone, and can be readily recovered and reused to prepare additional alkali metal alkoxide by reaction with alkali metal hydroxide according to the invention. Consequently, from an economic standpoint the only base consumed is alkali metal hydroxide, which offers significant savings over prior art sodium methoxide and other basic condensing agents.
More particularly, the present invention is directed to a method for the preparation of 1,3-diketones comprising the steps of:
(A) mixing an alkali metal base an aromatic hydrocarbon solvent and an alcohol having a boiling point of a least 100xc2x0 C. at atmospheric pressure;
(B) boiling the mixture and distilling water formed by the reaction between the base and the alcohol, whereby a solution of a hindered alkali metal alkoxide is formed in the solvent (alternatively, alkali metal alkoxide can be dissolved or suspended in an aromatic hydrocarbon);
(C) mixing a carboxylic acid ester with the solution of the alkali metal alkoxide in the aromatic hydrocarbon solvent;
(D) adding a ketone to the mixture and heating at a temperature in the range of 40-150xc2x0 C. until the formation of 1,3-diketone is substantially complete; and
(E) recovering 1,3-diketone from the reaction mixture.
The process of the present invention is a Claisen condensation comprising the following steps:
1) The base for the condensation, a hindered sodium or potassium alkoxide, is preferably prepared in situ before the actual condensation takes place. This can be advantageously carried out by boiling a mixture of a hindered alcohol, sodium or potassium hydroxide (pellets or aqueous solution), and an aromatic hydrocarbon solvent. The water formed is distilled. A slow stream of nitrogen is maintained over the mixture.
2) The temperature of the mixture is adjusted to a predetermined value. The ester component, preferably a methyl ester, is added to the mixture. The ketone component is then added slowly and the mixture is stirred until no more product is formed.
3) The mixture is preferably acidified using an excess of acid, which is then neutralized, whereupon the solvent is stripped. Depending on its nature, the hindered alcohol can be recovered by distillation, crystallization, or extraction with solvents. The diketone product, if it is a solid, can be purified by crystallization.
Alternatively, the sodium or potassium salt of the diketone can, in some cases, be isolated by filtration before acidifying the reaction mixture. The solid diketone salt can then be neutralized by slowly adding it with stirring to a mixture of water and an aromatic hydrocarbon. At the same time, an acid, such as acetic acid, is added to keep the pH neutral at all times. The organic layer can then be washed and the solvent stripped.
The 1,3-diketones prepared in accordance with this invention are preferably linear 1,3-diketones of general formula I 
wherein
R1 and R2 are independently selected from the group consisting of C1-C20 alkyl; phenyl; phenyl that is substituted by halogen, hydroxy, NO2, C1-C4 alkyl and/or C1-C4 alkoxy; C7-C9 phenylalkyl; and radicals of formula II
xe2x80x94Axe2x80x94Xxe2x80x94R4xe2x80x83xe2x80x83(II)
xe2x80x83wherein
A is selected from the group consisting of C1-C12 alkylene; C1-C12 alkylene that is substituted by hydroxy, halogen and/or alkoxy; phenylene; and phenylene that is substituted with halogen, hydroxy, NO2, C1-C4 alkyl and/or C1-C4 alkoxy;
X is oxygen or sulfur;
R4 is selected from the group consisting of hydrogen; C1-C18, alkyl; phenyl; phenyl that is substituted with halogen, hydroxy, C1-C4 alkyl, NO2 and/or C1-C4 alkoxy; and C7-C9 phenylalkyl; and
R3 is selected from the group consisting of hydrogen; C1-C20 alkyl; phenyl; phenyl that is substituted with halogen, hydroxy, C1-C4 alkyl, NO2, and/or C1-C4 alkoxy; and C7-C9 phenylalkyl;
wherein the process comprises a Claisen condensation of ketones of formula III 
with esters of formula IV 
wherein R5 is selected from the group consisting of C1-C5 alkyl; phenyl; and phenyl that is substituted with halogen, C1-C4 alkyl, or hydroxy;
or, if R2 in formula I is xe2x80x94(CH2)m OH, also with cyclic esters of formula V 
xe2x80x83wherein m is an integer of from 2 to 10.
In the above-described structural formulae, where R1 and/or R2 are C1-C20 alkyl, they may be linear or branched and are typically methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, and isomers and mixtures thereof. They are preferably C1-C18 alkyl, such as methyl, isopentyl, n-nonyl, pentadecyl or heptadecyl.
Where R1 and/or R2 are substituted phenyl, they preferably contain from 1 to 3, more preferably 1 or 2 substituents, most preferably one substituent.
Where R1 and/or R2 are (C1-C4 alkyl)phenyl, they are preferably phenyl substituted by 1 to 3, more preferably 1 or 2, alkyl groups, which are most preferably methyl groups. Typical examples include tolyl, xylyl, or mesityl.
Where R1 and/or R2 are halogen-substituted phenyl, they may be a phenyl ring that is substituted by one or more identical or different members selected from the group consisting of fluoro, chloro, and bromo, preferably chloro or bromo, and are typically chlorophenyl or dichlorophenyl.
Where R1 and/or R2 are C1-C4 alkoxy-substituted phenyl, they are typically methoxyphenyl, ethoxyphenyl, propoxyphenyl, butoxyphenyl, and isomers thereof.
Where R1 and/or R2 are C7-C9 phenylalkyl, they may be, for example, benzyl, phenylethyl, xcex1-methylbenzyl, 3-phenylpropyl, or xcex1,xcex1-dimethylbenzyl. Benzyl is preferred.
R1 and/or R2 are preferably C1-C18 alkyl, phenyl, (C1-C4 alkyl)phenyl, or xe2x80x94Axe2x80x94Xxe2x80x94R4.
Where A is C1-C12 alkylene, it can be either linear or branched, but is preferably linear, alkylene. Typical examples of such radicals include methylene, ethylene, propylene, butylene, pentylene, hexylene, heptalene, octalene, nonalene, decylene, undecylene, dodecylene, and isomers thereof. Alkylenes of from 1 to 6 carbon atoms are preferred, and n-propylene or n-pentylene are most preferred.
Where A is unsubstituted or substituted phenylene, it is preferably o- or p-phenylene, more preferably, A is unsubstituted phenylene.
Where R4 is C1-C18 alkyl, it may be linear or branched alkyl, as exemplified above in connection with R1 and R2 up to the corresponding number of carbon atoms.
Where R4 is substituted phenyl or C7-C9 phenylalkyl, it can have the same meanings as given for R1 and R2.
R4 is preferably hydrogen, C1-C18 alkyl, or phenyl.
Where R3 is C1-C20 alkyl, substituted phenyl, or C7-C9 phenylalkyl, it can have the same meanings as given for R1 and R2.
R3 is preferably hydrogen or C1-C4 alkyl, e.g., methyl, ethyl, propyl, butyl, or isomers thereof, but is more preferably hydrogen.
Where R5 is C1-C5 alkyl, it can be methyl, ethyl, propyl, butyl, butyl, pentyl, or isomers thereof. More preferably, R5 is C1-C4 alkyl, most preferably, methyl.
Where R5 is (C1-C4 alkyl)phenyl, it can have the same meanings as those given for R1 and R2.
As stated above, the basic condensing agent according to the invention is an alkali metal alkoxide of an alcohol, said alcohol having a boiling point of at least 100xc2x0 C. at atmospheric pressure. The alkali metal of the alkoxide is any one or more of the alkali metals. Sodium and potassium alkoxides are preferred. The alcohol with a boiling point of at least 100xc2x0 C. at atmospheric pressure can be primary, secondary, or tertiary, and can have 1-2 alcoholic hydroxyl groups. The alcohol can be aliphatic or cycloaliphatic, and can also be substituted with alkoxy groups having 1 to 5 carbon atoms and with aryl groups. Alkoxides of alcohols having a sterically hindered structure are particularly preferred. Preferred alkoxides are the sodium and potassium alkoxides of 1-butanol, 1-pentanol, cyclohexanol, 2-methyl-2,4-pentanediol, 2- propoxy-1- ethanol, 1-methoxy-2-propanol, 1-t-butoxy-2-propanol, 2-ethyl-1-hexanol, 1-phenylethanol, benzhydrol, triphenylmethanol, tert.-pentanol, 1-octanol, and 2-octanol, and the like.
The alkali metal alkoxide condensing agent can be prepared by reaction of the selected alcohol in the presence of aromatic hydrocarbon with such source of metal base as alkali metal or alkali metal hydride, with displacement of hydrogen; alkali metal amide, with displacement of ammonia; and lower alkali metal alkoxide, with displacement of lower alcohol. The alkali metal alkoxide condensing agent can also be prepared by dissolving a separately prepared alkali metal alkoxide of the selected alcohol in an aromatic hydrocarbon. In a particularly preferred emobidment, the alkali metal alkoxide is prepared according to the invention by reaction of the selected alcohol with alkali metal hydroxide, preferably sodium or potassium hydroxide, as the alkali metal base, with displacement of water. This can be advantageously carried out by heating a mixture of the selected alcohol, alkali metal hydroxide, and aromatic hydrocarbon solvent such that the water formed is distilled azeotropically, while maintaining a slow stream of nitrogen over the reaction mixture.
As disclosed above, ketones of formula III 
are employed as starting materials in the process of the present invention. Ketones that can be employed include aliphatic-aromatic, aliphatic-aliphatic, and cycloaliphatic ketones, such as acetophenone, acetone, methyl ethyl ketone, methyl n-propyl ketone, diethyl ketone, hexanone-2, pinacolone, di-n-propyl ketone, diisopropyl ketone, di-n-amyl ketone, chloroacetone, s-dichloroacetone, cyclohexanone, cyclopentanone, and the like.
As also disclosed above, another of the starting materials employed in the practice of the present invention are esters of formula IV 
wherein R5 is selected from the group consisting of C1-C5 alkyl; phenyl; and phenyl that is substituted with halogen, C1-C4 alkyl, or hydroxy; or, if R2 in formula I is xe2x80x94(CH2)m OH, also with cyclic esters of formula V 
wherein m is an integer of from 2 to 10. Such esters include, for example, aromatic esters, such as methyl benzoate, and aliphatic esters, such as ethyl acetate, methyl esters of linear C8 to C18 carboxylic acids, such as methyl stearate, methyl palmitate, methyl caprylate, mixtures of aliphatic esters, and the like.
Alcohols, e.g., methanol, can, if desired, be removed from the reaction mixture by distillation during the condensation, although it was observed in several cases that this might lead to transesterification and a decrease in overall yield.
In one preferred aspect of the present invention, aromatic beta-diketones can be prepared.
Exemplary of such aromatic beta-diketones are dibenzoylmethane, benzoyl 2,4-methylenedioxy benzoylmethane; benzoyl 3,5-dimethylbenzoyl methane; benzoyl 3-methylbenzoylmethane; benzoyl 4-methyl benzoylmethane; 3-methylbenzoyl 4-methylbenzoylmethane; benzoyl 4-chlorobenzoylmethane; benzoyl 2-bromobenzoylmethane; benzoyl 3,5-dichlorobenzoylmethane; benzoyl 2-nitrobenzoylmethane; benzoyl-2,3,4-trimethylbenzoylmethane; benzoyl-2,3,5-trichlorobenzoylmethane; benzoylstearoylmethane; 3-methylbenzoylstearoylmethane 3,4-dichlorobenzoylstearoylmethane benzoyl heptadecanoylmethane; 3-methylbenzoyltetradecanoylmethane; 4-chloro-nonadecoylmethane; 2-methylbenzoylauroylmethane; 3-nitrobenzoylmyristoylmethane; 2,3-ethoxybenzoyl palmitoylmethane; 2-methoxy benzoylstearoylmethane, 3-methylthio benzoyl 2,3-butylmethane and the like.
In order to prepare these and other beta-diketones, a corresponding acetophenone-type reactant is preferably selected. Exemplary of such useful acetophenone reactants are acetophenone; o-, m-, or p-methylacetophenone; o-, m-, or p-methoxy acetophenone; o-, m-, or p-methylthioacetophenone, o-, m-, or p-nitroacetophenone; 3,4-(methylenedioxy) acetophenone, o-, m-, or p-chloroacetophenone; o-, m-, or p-bromoacetophenone; 2,4-diethylacetophenone; 2,3,5-trichloroacetophenone, 2,3-dibromoacetophenone; 2,4-dimethoxyacetophenone; 2,4-propoxyacetophenone; 2,3-dimethylthioacetophenone; and the like.
Similarly, the ester reactant is selected on the basis of the identity of the desired betaketone. For example, an ester of benzoic acid can be used. Since the condensation reaction has as its by-product an alcohol formed from the ester group, normally there is no benefit from using higher alkyl esters. Accordingly, lower alkyl esters of benzoic acid are satisfactory, although higher esters can be used, if desired. In this description of the invention, the term xe2x80x9clowerxe2x80x9d means alkyl groups having up to about 5 carbon atoms.
Exemplary of such benzoate reactants are methylbenzoate; ethyl benzoate; propylbenzoate; butylbenzoate; pentylbenzoate; methyl o-,m- or p-methylbenzoate; ethyl o-, m-, or p-chlorobenzoate; methyl o-, m-, or p-methylthiobenzoate; ethyl o-, m-, or p-methoxybenzoate; methyl o-, m-, or p- bromobenzoate; ethyl o-, m-, or p- nitrobenzoic acid; ethyl 2,3-dimethyl benzoate; propyl 2,5-diethylbenzoate; ethyl 2,3,4-tri-methylbenzoate; butyl2,5-diethylthiobenzoate; ethyl 3,4-dimethoxybenzoate; methyl 2,3-dichlorobenzoate; ethyl 2,4-dibromobenzoate; propyl 2,3,5-trichlorobenzoate; propyl 2,4-diethyoxybenzoate and the like.
The condensation reaction can be performed at various elevated temperatures. In order to optimize the yield and purity of a beta-diketone product, temperatures between about 40xc2x0 C. and about 170xc2x0 C., preferably about 90xc2x0 C. to about 120xc2x0 C., are normally used. At these temperatures maximum amounts of high quality beta-diketone products can be recovered by standard procedures.
In order to obtain high quality product in high yield, it is desirable to use a molar excess of the ester reactant. This causes the reaction to proceed to about its theoretical maximum. The preferred excess of ester reactant useful in this process varies somewhat with the identity of the ester reactant. Since the use of more than the necessary amount of ester reactant will often increase the loss of this reactant, it is preferred to maintain the ratio of ester reactant to acetophenone below about 8:1, more preferably below about 2:1. Ratios of the ester of benzoic acid to acetophenone of about 2:1 will normally produce optimum yields and purity of dibenzoylmethane. The excess ester reactant can be recycled with the solvent so as to limit the reactant needs and reduce the cost of operating the process.
Aromatic hydrocarbons are preferably used as solvents for performing the process of the present invention. Since the temperature of the reaction is between about 100xc2x0 C. and about 170xc2x0 C., it is desirable to use an aromatic hydrocarbon solvent having a boiling point within this temperature range so that the reaction can proceed at atmospheric pressure; however, aromatic hydrocarbons having lower or higher boiling points can be used by adjusting the pressure accordingly. Among the aromatic hydrocarbons useful as solvents in the practice of the process of the present invention are ethyl benzene, cymene, diethylbenzene, dimethylethylbenzene, amyltoluene, toluene, trimethylbenzene, cumene, tetralin, xylenes, and the like.
At the end of the reaction, the 1,3-diketone is present in the reaction mixture in the form of an alkali metal salt. In some cases the salt can be isolated from the reaction mixture by filtration and used as is or converted to other 1,3-diketone metal salts, for example aluminum, calcium, magnesium, and zinc salts, by reaction with a compound of the selected metal such as aluminum sulfate, calcium and magnesium chlorides, and zinc acetate.
The solid dikidetone salt can also be neutralized to afford the free 1,3-diketone by slowly adding it with stirring to a mixture of water and an aromatic hydrocarbon while adding an acid, such as acetic acid, to keep the pH neutral. The organic layer can then be washed and the solvent stripped.
Alternatively, the entire reaction mixture can be acidified with an excess of acid, which is then neutralized, and the solvent is stripped. The alcohol reactant can be recovered together with the solvent or separately, by such techniques as distillation, crystallization or extraction with solvents as appropriate. The diketone product, if it is a solid, can be purified by crystallization.